1. Field of the Invention
The present invention relates to a method of feeding alumina and an apparatus therefor. More particularly, the present invention relates to an automatic feeding of alumina onto a freeze in Soderberg type aluminium electrolytic cell.
2. Description of the Prior Art
When feeding alumina into a Soderberg type aluminium electrolytic cell, a method of feeding alumina directly onto a freeze formed surrounding an anode from a moving type of alumina supplier provided outside the electrolytic cell has hitherto been used. FIG. 1 is a vertical section showing schematically a state of the electrolytic cell being operated by such method. In the figure, (1) is an anode, (2) is an anode casing, (3) is a H-figure beam for preventing the expansion of the anode, (4) is a gas collecting hood, (5) is an electrolytic bath, (6) is a molten metal, (7) is a cathode block, (8) is a freeze, (9) is alumina, (10) is a grand ramming paste, (11) is a working stand, (12) is a fin of the H-figure beam, (13) is an outer casing of cathode cell, and (14) is a cathode lining comprising firebricks and/or side carbons. Alumina is supplied onto the freeze (8) by a moving type of alumina supplier (not shown in drawing) provided outside of the electrolytic cell and accumulates there. The alumina (9) on the freeze (8) is charged into the eletrolytic bath (5) by breaking the freeze (8) using a freeze breaking means (not shown in drawing) periodically or in response to a signal indicating the lowering of alumina concentration in the electrolytic bath.
In the operation of the aluminium electrolytic cell an optimum value exists in the alumina concentration in the electrolytic bath, and, therefore, if the alumina concentration can be maintained within a certain narrow range, it can be connected to the improvement of current efficiency and the drop of cell voltage. The alumina concentration in the electrolytic bath increases temporarily by charging of alumina into the bath by breaking of the freeze and thereafter decreases almost linearly till the next breaking of the freeze. The increase in the alumina concentration at the time of breaking the freeze depends on the thickness of alumina layer accumulated on the freeze if the area of freeze broken is maintained constant, and, therefore, it is determined by the amount of alumina supplied at a time by the alumina supplier. Accordingly, in order to lessen the fluctuation of alumina concentration in the electrolytic bath it is necessary to reduce the amount of alumina supplied at a time as much as possible and to carry out the supply of alumina and breaking of freeze at short time intervals.
However, in the above described conventional alumina feeding method, there are limits to the reduction of the time intervals of supplying alumina since the feeding of alumina to multiple electrolytic cells is carried out by one alumina supplier provided outside of the electrolytic cells. And it is considerably difficult to distribute alumina uniformly onto the freeze, and it is inevitable that the amount of alumina charged into the bath at the time of breaking of the freeze becomes considerably random.
As the result of studies aimed at solving the defects in the above described conventional methods and providing an improved method of feeding alumina in which the prescribed amount of alumina can be always fed automatically onto the freeze and an apparatus therefor, the present inventor has found that alumina can be preferably fed by forming a specific shape of box body on the side of anode in the Soderberg type aluminium electrolytic cell and has accomplished the present invention.